I've designed lithium cells into products, and one of the issues is cell puncture. Flat cells with a plastic cover are subject to this. After the cover loses integrity, the cell absorbs moisture, the lithium reacts with the moisture to release hydrogen and the cell swells up.
The heat of the process is always of concern, so we would remove such a cell, place it in metal can and discharge it slowly through a resistor.
I can't say whether they were safe afterwards, but they were safer.